From 870c49e2a23bded9870c0bec6d94b75ca3d30662 Mon Sep 17 00:00:00 2001
From: Alex Olivas <alex.r.olivas@gmail.com>
Date: Wed, 30 Dec 2020 15:39:01 -0700
Subject: [PATCH] Moving from an empty peg is also an illegal move, but the
 code that checks this segfaults.

---
 epi_judge_cpp/hanoi.cc | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/epi_judge_cpp/hanoi.cc b/epi_judge_cpp/hanoi.cc
index c6d076b23..12501db87 100644
--- a/epi_judge_cpp/hanoi.cc
+++ b/epi_judge_cpp/hanoi.cc
@@ -25,10 +25,11 @@ void ComputeTowerHanoiWrapper(TimedExecutor& executor, int num_rings) {
 
   for (const vector<int>& operation : result) {
     int from_peg = operation[0], to_peg = operation[1];
-    if (!pegs[to_peg].empty() && pegs[from_peg].top() >= pegs[to_peg].top()) {
+    if (pegs[from_peg].empty() || (!pegs[to_peg].empty() &&
+				   pegs[from_peg].top() >= pegs[to_peg].top())) {
       throw TestFailure("Illegal move from " +
-                        std::to_string(pegs[from_peg].top()) + " to " +
-                        std::to_string(pegs[to_peg].top()));
+                        std::to_string(from_peg) + " to " +
+                        std::to_string(to_peg));
     }
     pegs[to_peg].push(pegs[from_peg].top());
     pegs[from_peg].pop();